Process for producing optical information recording medium and optical information recording medium produced by the process

ABSTRACT

An optical information recording medium which can eliminate the necessity for the initialization process. A crystallization assisting layer ( 3 ) comprising a given material is formed over a substrate ( 1 ) on one side thereof, and a recording layer ( 4 ) comprising a Ge—Sb—Te alloy is formed directly on the layer ( 3 ). Since the recording layer ( 4 ) crystallizes immediately after film formation, no initialization process is necessary for the optical information recording medium obtained. Examples of the material of the crystallization assisting layer firstly include materials having a face-centered cubic lattice system crystal structure. Examples thereof secondly include tellurium-free materials having a rhombohedral lattice system crystal structure. An especially preferred crystallization assisting layer is a discontinuous island-like film made of a material comprising bismuth and/or a bismuth compound. Incorporation nitrogen into the crystallization assisting layer provides an optical information recording medium which need not be initialized and has excellent overwrite cyclability.

TECHNICAL FIELD

The present invention relates to phase change optical informationrecording mediums having a recording layer of changing phase between acrystalline state and an amorphous state in accordance with theintensity of an irradiation beam, and in particular, it relates to amethod for producing an optical information recording medium capable ofmaking an initialization process unnecessary.

BACKGROUND ART

Recently, optical information recording mediums have been extensivelystudied and developed as means for recording, reading and erasing animmense quantity of information. Especially, a so-called phase changeoptical disk which records/erases information, using the fact that thephase of the recording layer changes reversibly between a crystallinestate and an amorphous state, has the advantage that only by changingthe laser beam power, old information is erased while new information isbeing recorded simultaneously (hereinafter referred to as “overwrite”).Thus, such optical disk is regarded as being full of promise.

As the recording materials of such overwritable phase change opticaldisk, chalcogen alloys are mainly used which include In—Se alloys (see“Appl. Phys. Lett. Vol. 50, p. 667, 1987”), In—Sb—Te alloys (see “Appl.Phys. Lett. Vol. 50, p.16, 1987”), and Ge—Te—Sb alloys (see JapanesePatent Laid-Open Publication Sho No. 62-53886), which have a low meltingpoint and a high absorption efficiency for a laser beam.

When information is actually recorded/erased on/from such optical diskof a chalcogen alloy, at least one kind of dielectric layer of amaterial selected from the group consisting of metal or semi-metaloxides, carbides, fluorides, sulfides, and nitrides is generally formeddirectly above and/or under the recording layer in order to prevent thesubstrate from being deformed due to heat produced on recording/erasing,to prevent the recording layer from being oxidized, and/or to preventthe substances from moving along the guide grooves or from beingdeformed.

Optical disks having a three- or four-layered structure which includes arecording layer of a chalcogen alloy, a dielectric layer provideddirectly under and/or above the recording layer, a reflective layerwhich also acts as a cooling layer (for example, Al-alloy) provided onan opposite side of a transparent substrate from the recording layer,provided on the substrate, are the mainstream of the phase changeoptical disks because they are preferable in terms of therecording/erasing characteristics.

In general phase change optical disks, when the recording layer isirradiated with a laser beam having a recording power to heat it up toits melting point and is then rapidly cooled, the recording layermaterial is produced amorphous to thereby form a recording mark. Then,when the recording layer is irradiated with a laser beam having anerasing power to be heated to more than the crystallizing temperatureand then gradually cooled, the recording layer material is crystallizedto thereby erase the recording mark.

Such phase change optical disks are each produced by sequentiallyforming thin layers as the respective layers on the substrate bysputtering/evaporation. Since the recording layer present immediatelyafter its layer formation is amorphous, it is irradiated with a laserbeam to be wholly crystallized, which is generally referred to as“initialization process”, and the optical disks, thus obtained, are thenshipped.

However, this initialization process takes a time of a little less thanone minute to initialize the whole optical disk having a diameter of 120mm even with the use of the most-efficient laser beam irradiation, whichleads to an increase in the manufacturing cost of the optical disks. Forthe time required for processing one optical disk in each manufacturingsubstep (cycle time), the time required for the initialization processis long compared to the substrate molding step or the layer formingstep. Thus, in order to eliminate a time loss taken to pass to theinitialization process when the cycle time for the layer forming step is8 seconds, the six or seven very expensive initializing devices arerequired. As a result, by performing the initialization process, themanufacturing cost of the optical disks is increased.

In order to reduce the time required for the initialization process, forexample, Japanese Patent Laid-Open Publication Hei No. 5-342629discloses providing an auxiliary layer of an easily crystallizablecontinuous film or discontinuous island-like film adjacent to therecording layer. As the components of the auxiliary layer, compounds arenamed which include tellurium (Te), Selenium (Se) or Te—Se compounds.

However, according to this method, the time required for initializingthe recording layer is reduced, but the initialization process cannot beeliminated as a rule, excluding the case where both of the auxiliarylayer and the recording layer are comprised of extremely easilycrystal-growing substances.

It is therefore an object of the present invention to provide an opticalinformation recording medium which eliminates the necessity for theinitialization process.

DISCLOSURE OF THE INVENTION

The present invention provides a method for producing an opticalinformation recording medium which has on one side of a substrate arecording layer whose main components comprise germanium (Ge), antimony(Sb) and tellurium (Te) (hereinafter referred to as “Ge—Te—Sb alloy ”),comprising the steps of forming a crystallization assisting layer ofmaterials having a face-centered cubic lattice system crystal structureon one side of the substrate, and forming a recording layer directlyabove the crystallization assisting layer. According to this method, therecording layer immediately after its formation is crystallized.

The Ge—Te—Sb alloys take two types of crystal phases: namely, aface-centered cubic lattice system crystal structure and a hexagonalsystem crystal structure. It is known that as the temperature of thisalloy is raised from its amorphous state, its phase changes from aface-centered cubic lattice crystal structure to a hexagonal structure.In the present invention, the recording layer is easily crystallizedwhen its layer is formed due to the presence of the crystallizationassisting layer having the same face-centered cubic lattice systemcrystal structure as the recording layer.

The face-centered cubic lattice system crystal structures includeface-centered cubic lattices, and face-centered tetragonal lattice;diamond-shape structures: CuAu—, CuPt—, Ni₂Cr—, Cu₃Au—, Ni₄Mo—, Ag₃Mg—,Ni₃V—, Cu₃Pd—, and Au₃Mn-type superlattices; NaCl—, NaTl—, ZnS—, CaF₂—,FeS₂—, cristobalite high-temperature-, Laves phase MgCu₂—, Cu₃Au—,Al₃Ti—, Cu₂AlMn—, Al₂MgO₄—, and Bi₂Te₃-type structures; and theirinterstitial and substitutional solid solutions.

The present invention also provides a method for producing an opticalinformation recording medium having on one side of a substrate arecording layer whose main components comprise germanium (Ge), antimony(Sb) and tellurium (Te), comprising the steps of forming on one side ofa substrate a crystallization assisting layer of a tellurium (Te)-freematerial having a crystal structure of a rhombohedral lattice system,and forming a recording layer directly over the crystallizationassisting layer. According to this method, the recording layer becomescrystallized immediately after its formation.

In the present invention, the absolute value of a lattice unconformitybetween the crystal structure of the crystallization assisting layer andthat of the recording layer is preferably not more than 8%. The latticeunconformity is represented by:

Lattice unconformity (%)=((B−A)/A)×100  (a)

A: When the recording layer is of a face-centered cubic lattice systemcrystal, an atomic interval in a direction <110> of the crystal;

B: A particular one of the atomic intervals of a crystallizedcrystallization assisting layer such that the difference between A andthe particular atomic interval B is minimum among the differences eachbetween A and a respective one of the atomic intervals of thecrystallized crystallization assisting layer. In the case of theface-centered cubic lattice system, it is generally the atomic intervalin a direction <100> or <110>.

When the crystal comprises two or more kinds of elements, the distancebetween two adjacent atoms of different kinds may be used as an atomicinterval in the expression (a). When A is greatly different from B inthe expression (a), the atomic interval B of the crystallizationassisting layer may be assumed to be an integer or fraction times theatomic interval.

The range of lattice unconformity is preferably −4.5 to +8%, and morepreferably −3 to +7%.

Examples of materials each of which has a crystal structure of aface-centered cubic lattice system where an absolute value of latticeunconformity between the crystal structure of that material and that ofthe recording layer is not more than 8% include PbTe and Bi₂Te₃.

Examples of tellurium-free materials each of which has a crystalstructure of a rhombohedral system where an absolute value of latticeunconformity between the crystal structure of that material and that ofthe recording layer is not more than 8% include antimony (Sb), bismuth(Bi), antimony (Sb) compounds, and bismuth (Bi) compounds. The Sbcompounds include Sb alloys, and intermetallic compounds of Sb and othermetals or semimetals. The Bi compounds include Bi alloys, andintermetallic compounds of Bi and other metals or semimetals.

In the present invention, the thickness of the crystallization assistinglayer is preferably not more than 200 Å. If the thickness is larger than200 Å, the record erasing characteristics would be deteriorated. Thethickness of the crystallization assisting layer is more preferably notmore than 100 Å. If this layer is excessively thin, the recording layercan be crystallized insufficiently. Thus, it is preferably not less than1 Å.

In the present invention, the crystallization assisting layer may be inthe form of a continuous film or a discontinuous island-like film, whichis contacted with the recording layer. Most preferably, it is adiscontinuous island-like film of materials which contain bismuth (Bi)and/or a bismuth (Bi) compound.

The optical information recording medium whose recording layer iscrystallized by the crystallization assisting layer provided so as to becontacted with a substrate-side surface of the recording layereliminates the necessity for the initialization process. If a continuousfilm of materials which comprise bismuth (Bi) and/or a bismuth (Bi)compound is used as the crystallization assisting layer, the CNR(Carrier to Noise-Ratio) in the second or subsequent recording byoverwriting is slightly lower than that in the first recording.

In comparative examples, if a discontinuous island-like film ofmaterials containing bismuth (Bi) and/or a bismuth (Bi) compound is usedas the crystallization assisted layer, the CNR in the second orsubsequent recording by overwriting is substantially the same as that inthe first recording.

The discontinuous island-like film is formed, for example, by sputteringsuch that its thickness is not more than a predetermined value.

When the crystallinity of the recording layer formed on thecrystallization assisting layer is insufficient, the recording layer ispreferably formed by setting the temperature of the substrate in a rangeof from 45° C. through a temperature inclusive above which temperaturethe substrate would be deformed (at 110° C. when the substrate isproduced of polycarbonate). Thus, the recording layer is placed in astabilized crystalline state.

The methods for maintaining the substrate at high temperatures underformation of the recording layer include (1) heating the substrate orthe crystallization assisting layer which underlies the recording layerimmediately before the formation of the recording layer to therebymaintain the substrate at high temperature; (2) starting to heat thesubstrate or the crystallization assisting layer after the formation ofthe recording layer has started and continuing to heat the substrate orthe crystallization assisting layer during the formation of therecording layer; (3) starting to heat the substrate or the layer surfaceof the recording layer immediately after the recording layer has beenformed; and (4) starting to form the recording layer immediately afterthe preceding layer has been formed, using the heat produced by theformation of the preceding layer and stored within the substrate.

The heating methods include irradiating a surface of the layer formed onthe substrate (a surface of the crystallization assisting layer) withlight including heat rays; and heating a substrate holder itself with aheater or the like, or using high frequency induction, flash exposure;or plasma processing.

In the producing method of the present invention, the formation of thecrystallization assisting layer is preferably performed within a layerforming atmosphere to which a nitrogen gas is added.

When the producing method of the present invention includes the step offorming a first dielectric layer between the substrate and thecrystallization assisting layer and/or the step of forming a seconddielectric layer on an opposite side of the recording layer from thecrystallization assisting layer, the formation of the first and/orsecond dielectric layer is preferably performed within a layer formingatmosphere to which a nitrogen gas and/or an oxygen gas is added.

The present invention also provides an optical information recordingmedium with a recording layer formed on one side of a substrate, therecording layer comprising materials whose main components are germanium(Ge), antimony (Sb) and tellurium (Te), wherein the recording layer isformed in a crystalline state and wherein the crystallization assistinglayer is formed in contact with the substrate side surface of therecording layer, the recording layer and the crystallization assistinglayer being produced by the respective above-mentioned producingmethods.

In the inventive optical information recording medium, a ratio x, y, andz of the respective elements (Ge, Sb, Te) of the main components of therecording layer (Ge:Sb:Te=x:y:z where x+y+z=1) is preferably in a rangeshown hatched in a triangular graph of FIG. 2, which satisfies thefollowing expressions (1)-(3) simultaneously:

0.1≦x≦0.4  (1)

0.08≦y  (2)

0.45≦z≦0.65  (3)

When x<0.1, the optical information recording medium is not preferablein terms of stability. When x>0.4, y<0.08, z<0.45 and z>0.65, theseconditions are unpreferable because the recording layer is difficult tocrystallize.

A preferable range of the ratio x, y and z of the respective elements ofthe main components of the recording layer (Ge:Sb:Te=x:y:z wherex+y+z=1) should satisfy the following expressions (4)-(6)simultaneously:

0.15≦x≦0.3  (4)

0.12≦y  (5)

0.5≦z≦0.6  (6)

The materials of the recording layer are preferably Ge—Te—Sb—Bi alloyscontaining Bi in addition to Ge, Te and Sb. The materials may beGe—Te—Sb or Ge—Te—Sb—Bi alloys, for example, containing hydrogen,nitrogen, oxygen, carbon, Al, Ti, Fe, Co, Ni, Cu, Zn, Ga, Se, Sn, In,Ag, Pd, Rh, Ru, Mo, Nb, Hf; Zr, Ta, W, Re, Os, Ir, Pt, Au, Tl and/or Pb.Those elements may be added from the target during the formation of therecording layer or added in a gaseous state to the atmosphere gas so asto be contained within the recording layer.

In the inventive optical information recording medium, the thickness ofthe recording layer is preferably 50-1000 Å. If it is less than 50 Å,the recording layer could not obtain a satisfactory recordingsensitivity. When it exceeds 1000 Å, a problem about the recordingsensitivity and resolution would occur, undesirably.

The inventive optical information recording medium preferably has a4-layered structure in which a crystallization assisting layer, arecording layer, a dielectric layer and a reflective layer are formed onthe substrate in this order. More preferably, the inventive opticalinformation recording medium has a 5-layered structure in which a firstdielectric layer, a crystallization assisting layer, a recording layer,a second dielectric layer and a reflective layer are formed on thesubstrate in this order. The inventive optical information recordingmedium may further include other necessary layers additionally.

As the first and second dielectric layers, materials having high heatresistance and a melting point of not less than 1000° C. are preferable;for example, SiO₂; a mixture of ZnS and SiO₂; Al₂O₃; AlN; and Si₃N₄.Although the thickness of the first dielectric layer is not especiallyspecified, the thickness of the second dielectric layer is preferably50-500 Å. If it is less than 50 Å, it can not provide a satisfactoryrecording sensitivity. If it exceeds 500 Å, it cannot providesatisfactory overwrite cyclability. The thickness of the reflectivelayer is preferably not less than 300 Å.

The methods of forming the respective layers include evaporation,sputtering and ion plating.

A method of confirming the presence of the crystallization assistinglayer in the optical information recording medium will be describednext.

The first method is to observe a cross section of the opticalinformation recording medium, with a transmission electron microscope.The elements of the crystallization assisting layer can be specifiedwith the aid of an electron beam diffraction apparatus and an energydispersion X-ray analysis apparatus. When the crystallization assistinglayer is island-like or very thin, it is difficult to confirm itspresence, using this method.

The second method includes slowly etching layers formed on the substrateof the optical information recording medium, by sputtering, in adirection perpendicular to the substrate surface while analyzingelements present at respective positions in the layers formed on thesubstrate, using a secondary ion mass spectrometry (SIMS) or Augerelectron spectroscopy (AES). This method is effective when thecrystallization assisting layer is island-like or very thin.

According to this method, as the recording layer is slowly etched towardan interface between the recording layer and the crystallizationassisting layer while the elements are being analyzed, the quantity ofelements which compose the crystallization assisting layer increasestoward the interface between the crystallization assisting layer and itsunderlying layer (generally, the dielectric layer), and after theinterface is reached, rapidly decreases. By finding this phenomenon, thepresence of the crystallization assisting layer will be known.

As an example, in the case of an optical disk which has a layerstructure of a substrate/a first dielectric layer/a crystallizationassisted layer/a recording layer/a second dielectric layer/a reflectivelayer/a UV set resin layer, a method of confirming the presence of thecrystallization assisting layer, using the second method, will beexplained as follows. First, an adhesive tape is adhered to the UV setresin layer to thereby separate the layered layer from the substrate. Atthis time, since the second dielectric layer is generally separated fromthe recording layer, the substrate on which the recording layer and thecrystallization assisting layer remain is put into the secondary ionmass spectrometry or Auger electron spectroscopy to analyze the elementsusing by etching slowly from the recording layer side.

In the case of an optical disk where the recording layer comprises Ge,Te and Sb and where the crystallization assisting layer is in the formof a discontinuous island-like film of Bi, the presence of Ge, Te and Sbis first confirmed by analysis of the elements. As the layers on thesubstrate are further etched, the presence of Bi is recognized. Byfurther etching, the quantity of Bi increases whereas the quantities ofGe, Te and Sb decrease gradually. When the dielectric layer is reached,no presence of Ge, Te, Sb and Bi is recognized. Thus, when suchphenomenon is found in the second method, it can be determined that theoptical disk comprises a crystallization assisting layer comprised of adiscontinuous island-like film of Bi.

An optical disk whose recording layer is crystallized immediately afterits formation is easily distinguished from an optical disk whoserecording layer is crystallized in the initialization process, with thefollowing method.

In the optical disk whose recording layer is crystallized by aninitializing device using general laser-beam irradiation, the innermostand outermost peripheries of the recording layer are not initialized toremain amorphous state due to the composition of the initializingdevice. Thus, the innermost and outermost peripheries of the disk andits intermediate portion are different in reflectivity, which will bevisually recognized by those skilled in the art. In comparativeexamples, in the optical disk whose recording layer is crystallizedimmediately after its formation and not subjected to the initializationprocess, there is no such difference in reflectivity because the wholesurface of the recording layer is crystallized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a layer structure of an opticalinformation recording medium corresponding to one embodiment of thepresent invention;

FIG. 2 is a triangular graph indicative of a preferable range of a ratioof the respective elements of Ge—Sb—Te as the main components of therecording layer;

FIG. 3 is a graph of diffraction X-ray spectra obtained by X-raydiffraction from X-ray diffraction samples A-C of a second embodiment;

FIG. 4 is a photograph of a thin layer corresponding to acrystallization assisting layer of an example 3-1;

FIG. 5 is a photograph of a thin layer corresponding to acrystallization assisting layer of an example 3-2; and

FIG. 6 is a graph of diffraction X-ray spectra obtained by X-raydiffraction from X-ray diffraction samples A-G of a third embodiment.

BEST MOOD FOR CARRYING OUT THE INVENTION

First Embodiment

Phase change optical disks each having a layer structure of FIG. 1 wereproduced as follows:

First, as each substrate 1, a polycarbonate substrate with a center holewas prepared, having a diameter of 120 mm, a thickness of 0.6 mm, and ahelical guide groove of 0.74 μm in width and 1.4 μm in track pitch.

Then, formed on this substrate 1 were a first dielectric layer 2 of amixture of ZnS and SiO₂ (the content of SiO₂ is 30 mole %) and having athickness of 2500 Å, a crystallization assisting layer 3 produced ofeach of materials shown in Table 1 and having a corresponding thicknessalso shown in Table 1, a recording layer 4 of a Ge—Te—Sb alloy having athickness of 200 Å, a second dielectric layer 5 of a mixture of ZnS andSiO₂ (the content of SiO₂ is 30 mole %) having a thickness of 150 Å, anda reflective layer 6 of an Al—Ti alloy (the content of Ti is 1.1 atom %)having a thickness of 800 Å in this order. In a comparative example 1-1,no crystallization assisting layer 3 was formed. A UV cured resin layer7 was formed on the reflective layer 6.

The formation of the first and second dielectric layers 2 and 5 wasperformed by RF sputtering, using a mixture of ZnS and SiO₂ (the contentof SiO₂ was 30 mole %) as the target. An argon gas was used as theatmosphere gas for sputtering.

The formation of the crystallization assisting layer 3 was performed byDC sputtering, using each of the materials whose purity was 99.9 atom %as the target. An argon gas was used as the atmosphere gas.

The formation of the recording layer 4 was performed by DC sputtering,using a Ge—Te—Sb alloy as the target. An argon gas was used as theatmosphere gas. Analysis of the recording layer 4 by a fluorescent X-rayclarified that the composition of the formed recording layer was: Ge=21atom %, Te=24 atom %, and Sb=55 atom %.

The formation of the reflective layer 6 was performed by DC sputtering,using an Al—Ti alloy (the contents of Ti was 1.1 atom %) as the target.An argon gas was used as the atmosphere gas.

The formation of the UV cured resin layer 7 was performed byspin-coating the reflective layer 6 with a UV curable resin, and then byirradiating the resin with UV rays.

Table 1, attached hereto, shows the materials and thicknesses, crystalstructures, the values of B of the expression (a) (atomic intervals ofthe crystals), and the values of lattice unconformity calculated fromthe expression (a) of the crystallization assisting layers therespective examples and comparative examples.

The value of A of the expression (a) is 4.325 Å. When the crystalstructure has a hexagonal system, the value of B is one of the atomicinterval in the a-axial direction and the atomic interval in the c-axialdirection, which has the smaller one of the differences between A andthese atomic intervals. When the crystal structure has a body-centeredcubic lattice system, the value of B is one of the atomic interval in a<100> direction and the atomic interval in a <111> direction, which hasthe smaller one of the differences between A and these atomic intervals.When the crystal structure has a monoclinic system, the value of B isthe atomic interval between the closest atoms.

First, the reflectivity and recording characteristics (CNR (Carrier toNoise Ratio) and erase ratio) of each of examples 1-1 to 1-4 andcomparative examples 1-1 to 1-7 of uninitialized optical disks, thusobtained, were measured. Then, the reflectivity and recordingcharacteristics (CNR and erase ratio) of each example after itsinitialization were measured.

The measurement of the CNR and erase ratio of the disk of each examplewas performed after recorded once and by overwriting, 100 times. Themeasurement of the reflectivity and the recording characteristics wasperformed with the aid of a laser of a wavelength of 640 nm with anobjective lens having a NA of 0.6.

The initialization was performed by rotating the disk at a linearvelocity of 5 m/s and causing a laser beam of a wavelength of 820 nm toscan the same place on the disk ten times or more. The cross-sectionaloutline of the laser beam was an ellipse having a longer axis of 96 μmand a shorter axis of 1.5 μm. Thus, a flat area between two adjacentguide grooves was also initialized so as to have the same state as theguide grooves.

The measurement of the disk reflectivity was performed on guidegroove-free flat portions of each disk before and after itsinitialization. The measurement of the disk reflectivity after itsinitialization was performed after it was confirmed that the disk wassufficiently crystallized in the initialization process such that thereflectivity was saturated.

The measurement of the C/N ratio of each disk was performed after asingle signal whose mark and space lengths were 0.61 μm was recordedonce and after the same signal was overwritten 100 times during the diskrotation at a linear velocity of 6 m/s.

The measurement of the erase ratio was performed by measuring an amountof a decrease in the carrier wave of a signal whose mark and spacelengths were 0.61 μm after a signal whose mark and space lengths were0.61 μm was recorded once or overwritten 100 times and further a signalwhose mark and space lengths were 2.85 μm was overwritten once. Theresults of those measurements are together shown in Table 2 attachedhereto.

As will be seen from Table 2, the crystal structures of the materialswhich compose the crystallization assisting layers of the examples 1-1to 1-4 are of a face-centered cubic lattice system and the absolutevalue of the lattice unconformity is not more than 8%. Thus,satisfactory results were obtained before the initialization; that is,the C/N ratio was 45 dB or more, the erase ratio was 20 dB or more, andthe reflectivity was 15% or more. The difference between before andafter the initialization about the reflectivity, C/N ratio and eraseratio was small. There was no large difference between after recordedonce and 100 times about C/N ratio and erase ratio. Thus, the necessityfor the initialization process can be eliminated.

In comparative examples, the comparative examples 1-1 to 1-7 exhibitedno results which satisfied all of the conditions, that is; 45 dB or moreof the C/N ratio, 20 dB or more of the erase ratio, and 15% or more ofthe reflectivity as the characteristics before the initialization, butexhibited satisfactory results after the initialization. Thus, theinitializing step cannot be eliminated. The crystal structures of thematerials which compose the crystallization assisting layers of thecomparative examples 1-4 and 1-5 are of a rhombohedral lattice system,but contain Te. Thus, those results were obtained.

The comparative example 1-4 used materials of the crystallizationassisting layer which contains Te and Se. Preferably, Se should not beused because it has toxicity and is required to handle with care whenthe target is produced or the layer is formed. Thus, the composition ofthe comparative examples 1-4 is not preferable because the necessity forthe initialization process cannot be eliminated as well as the producingstep is complicated.

Second Embodiment

EXAMPLE 2-1

A phase change optical disk having the same layer structure as the FIG.1 one was produced as follows.

First, a first dielectric layer 2 having a thickness of about 1400 Å wasformed on a polycarbonate substrate 1 having a diameter of 90 mm and athickness of 0.6 mm with a guide groove for a laser beam by RFsputtering with the aid of a target of ZnS—SiO₂. A Sb layer having athickness of 50 Å was then formed as the crystallization assisting layer3 on the first dielectric layer 2 by sputtering with the aid of a targetof Sb.

A recording layer 4 having a thickness of 200 Å of Ge₂Te₅Sb₂ was formedon the crystallization assisting layer 3 by sputtering with the aid of atarget of a Ge—Te—Sb alloy. A second dielectric layer 5 having athickness of 200 Å was then formed on the recording layer 4 bysputtering with the aid of a target of ZnS—SiO₂. A reflective layer 6having a thickness of 1500 Å was then formed on the second dielectriclayer 5 by sputtering with the aid of a target of an Al alloy. A UVcurable resin was then spin-coated on the reflective layer 6 to be curedto thereby form a UV cured resin layer 7.

In the recording layer 4 of the phase change optical disk, the ratio x,y, and z of the respective quantities of the component elements Ge, Sband Te(Ge:Sb:Te=x:y:z where x+y+z=1)was 0.22:0.22:0.56, which satisfiedthe expressions (1)-(3) simultaneously.

Comparative Example 2-1

A phase change optical disk was produced in a manner similar to that inwhich the example 2-1 was produced, excluding that no crystallizationassisting layer 3 was formed.

EXAMPLE 2-2

A phase change optical disk having the same layer structure as the FIG.1 one was produced as follows:

A first dielectric layer 2 having a thickness of about 1400 Å was formedon a substrate 1 similar to that of the example 2-1 in a manner similarto that in which the example 2-1 was produced. A Sb layer having athickness of 65 Å was then formed as the crystallization assisting layer3 on the first dielectric layer 2 in a manner similar to that in whichthe example 2-1 was produced. Then, the crystallization assisting layer3 was irradiated with light (for five minutes at a power of 500 W) froma halogen lamp to further crystallize the crystallization assistinglayer 3.

A recording layer 4 of Ge₃₁Te₅₇Sb₁₂ having a thickness 225 Å was thenformed on the crystallization assisting layer 3 by sputtering with theaid of a target of a Ge—Te—Sb alloy. A second dielectric layer 5 ofZnS—SiO₂ having a thickness of 200 Å, and a reflective layer 6 of an Alalloy having a thickness of 1500 Å were then sequentially formed bysputtering on the recording layer 4 in a manner similar to that in whichthe example 2-1 was produced. A UV cured resin layer 7 was then formedand set on the reflective layer 6 as in the example 2-1.

In the recording layer 4 of the phase change optical disk, the ratio x,y, and z of the respective quantities of the component elements Ge, Sb,and Te was Ge:Sb:Te=x:y:z=0.31:0.21:0.57 where x+y+z=1, and satisfiedthe expressions (1)-(3) simultaneously.

EXAMPLE 2-3

A phase change optical disk having the same layer structure as the FIG.1 one was produced as follows:

A first dielectric layer 2 having a thickness of about 1400 Å was formedon a substrate 1 similar to that of the example 2-1 in a manner similarto that in which the example 2-1 was produced. A Bi layer having athickness of 50 Å was then formed as the crystallization assisting layer3 on the first dielectric layer 2 by sputtering with the aid of a targetof Bi. Then, the crystallization assisting layer 3 was irradiated withlight (for five minutes at a power of 500 W) from a halogen lamp tofurther crystallize the crystallization assisting layer 3.

A recording layer 4 of Ge₂₃Te₅₄Sb₂₃ having a thickness of 225 Å was thenformed on the crystallization assisting layer 3 by sputtering with theaid of a target of a Ge—Te—Sb alloy. A second dielectric layer 5 ofZnS—SiO₂ having a thickness of 200 Å, and a reflective layer 6 of an Alalloy having a thickness of 1500 Å were then sequentially formed bysputtering on the recording layer 4 in a manner similar to that in whichthe example 2-1 was produced. A UV cured resin layer 7 was then formedand set on the reflective layer 6 as in the example 2-1.

In the recording layer 4 of the phase change optical disk, the ratio x,y, and z of the respective quantities of the component elements Ge, Sb,and Te was Ge:Sb:Te=x:y:z=0.23:0.23:0.54 where x+y+z=1, and satisfiedthe expressions (1)-(3) simultaneously.

EXAMPLE 2-4

A phase change optical disk having the same layer structure as the FIG.1 was produced as follows:

A first dielectric layer 2 having a thickness of about 1400 Å was formedon a substrate 1 similar to that of the example 2-1 in a manner similarto that in which the example 2-1 was produced. A Bi layer having athickness of 50 Å was then formed as the crystallization assisting layer3 on the first dielectric layer 2 by sputtering with the aid of a targetof Bi.

A recording layer 4 of Ge₁₇Te₅₅Sb₂₈ having a thickness of 225 Å was thenformed on the crystallization assisting layer 3 by sputtering with theaid of a target of a Ge—Te—Sb alloy. A second dielectric layer 5 ofZnS—SiO₂ having a thickness of 200 Å, and a reflective layer 6 of an Alalloy having a thickness of 1500 Å were then sequentially formed bysputtering on the recording layer 4 in a manner similar to that in whichthe example 2-1 was produced. A UV cured resin layer 7 was then formedand set on the reflective layer 6 as in the example 2-1.

In the recording layer 4 of the phase change optical disk, the ratio x,y, and z of the respective quantities of the component elements Ge, Sb,and Te was Ge:Sb:Te=x:y:z=0.17:0.28:0.55 where x+y+z=1, and satisfiedthe expressions (1)-(3) simultaneously.

EXAMPLE 2-5

A phase change optical disk having the same layer structure as the FIG.1 was all produced in a manner similar to that in which the example 2-4was produced, excluding formation of a layer of Ge₃₇Te₅₄Sb₉ having athickness of 225 Å as the recording layer 4.

In the recording layer 4 of the phase change optical disk, the ratio x,y, and z of the respective quantities of the component elements Ge, Sb,and Te was Ge:Sb:Te=x:y:z=0.37:0.09:0.54 where x+y+z=1, and satisfiedthe expressions (1)-(3) simultaneously.

EXAMPLE 2-6

A phase change optical disk having the same layer structure as the FIG.1 was all produced in a manner similar to that in which the example 2-1was produced, excluding that the thickness of the first dielectric layer2 of ZnS—SiO₂ was 1200 Å; that the thickness of the crystallizationassisting layer 3 of Sb layer was 100 Å; that the thickness of therecording layer 4 of a Ge₂Te₅Sb₂ alloy was 250 Å; that the thickness ofthe second dielectric layer 5 of ZnS—SiO₂ was 150 Å; and that thethickness of the reflective layer 6 of an Al alloy was 500 Å.

An X-ray diffraction sample A was produced which comprised respectivelayers formed on a smooth glass substrate with each of the respectivelayers being exactly the same in structure as a corresponding one of thelayers of the last mentioned disk.

An X-ray diffraction sample B was also produced in a manner similar tothat in which the sample A was produced, excluding that the thickness ofa Sb layer as the crystallization assisting layer 3 having a thicknessof 180 Å was formed on the smooth glass substrate and that no layerscorresponding to the recording layer 4, reflective layer 6 and UV curedresin layer 7 were formed.

Comparative Example 2-2

A phase change optical disk was produced in a manner similar to that inwhich the example 2-6 was produced, excluding that no layercorresponding to the crystallization assisting layer 3 was formed.

An X-ray diffraction sample C was produced which comprises respectivelayers formed on a smooth glass substrate with each of the layers beingexactly the same in structure as a corresponding one of the layers ofthe last-mentioned disk.

EXAMPLE 2-7

A phase change optical disk having the same layer structure as the FIG.1 was all produced in a manner similar to that in which the example 2-4was produced, excluding formation of a layer of Ge₄₃Te₅₂Sb₄ having athickness of 225 Å as the recording layer 4.

In the recording layer 4 of the phase change optical disk, the ratio x,y, and z of the respective quantities of the component elements Ge, Sb,and Te was Ge:Sb:Te=x:y:z=0.43 (=43/99):0.04 (=4/99):0.53 (=52/99) wherex+y+z=1. The value of x was larger than an upper limit of the range ofthe expression (1) and the value of y was smaller than a lower limit ofthe range of the expression (2).

EXAMPLE 2-8

A phase change optical disk having the same layer structure as the FIG.1 was all produced in a manner similar to that in which the example 2-4was produced, excluding formation of a layer of Ge₂₅Te₄₀Sb₃₅ having athickness of 225 Å as the recording layer 4.

In the recording layer 4 of the phase change optical disk, the ratio x,y, and z of the respective quantities of the component elements Ge, Sb,and Te was Ge:Sb:Te=x:y:z=0.25:0.35:0.40 where x+y+z=1. The value of zwas smaller than a lower limit of the range of the expression (3).

EXAMPLE 2-9

A phase change optical disk having the same layer structure as the FIG.1 was all produced in a manner similar to that in which the example 2-4was produced, excluding formation of a layer of Ge₂₅Te₃₂Sb₄₃ having athickness of 225 Å as the recording layer 4.

In the recording layer 4 of the phase change optical disk, the ratio x,y, and z of the respective quantities of the component elements Ge, Sb,and Te was Ge:Sb:Te=x:y:z=0.25:0.43:0.32 where x+y+z=1. The value of zwas smaller than a lower limit of the range of the expression (3).

EXAMPLE 2-10

A phase change optical disk having the same layer structure as the FIG.1 was all produced in a manner similar to that in which the example 2-4was produced, excluding formation of a layer of Ge₂₀Te₆₇Sb₁₃ having athickness of 225 Å as the recording layer 4.

In the recording layer 4 of the phase change optical disk, the ratio x,y, and z of the respective quantities of the component elements Ge, Sb,and Te was Ge:Sb:Te=x:y:z=0.20:0.13:0.67 where x+y+z=1. The value of zis larger than an upper limit of the range of the expression (3).

Advantageous Effects Produced by the Crystallization Assisting Layer

The reflectivity and recording characteristics (CNR (Carrier to NoiseRatio) and erase ratio) of each of the optical disks of the examples 2-1to 2-10 and comparative examples 2-1 and 2-2, thus obtained, weremeasured before its initialization. They were also measured afterinitialization.

The initialization was performed by irradiating the respective diskswith a laser beam having wavelength of 680 nm, using “MO Disk BulkEraser LK101A” manufactured by K.K. Shibasoku. The measurement of therecording characteristics was performed as follows. First, a laser beamhaving a wavelength of 680 nm was modulated for each sample between anoptimal peak power and an optimal bias power. Each optical disk samplewhich was rotating at 1800 rpm was irradiated with the modulated laserbeam to perform initial recording with a first recording signal and theCNR was then measured. Next, the initial recording signal on the samplewas overwritten by a recording signal different from the first recordingsignal, and an erase ratio of the initial recording signal was thenmeasured.

In the samples of the examples 2-1 to 2-3, 2-8, 2-9 and the comparativeexample 2-1, the first and second recording signal frequencies were 1.08and 3.89 MHz, respectively. In the samples of the examples 2-4 to 2-7,2-10 and the comparative example 2-2, the first and second recordingsignal frequencies were 3.89 and 1.08 MHz, respectively.

The results of measurement of those characteristics are shown in Table 3attached hereto. In the samples of the examples 2-8 to 2-10, thereflectivity of the initial records on each disk before theinitialization was excessively low and the CNR and erase ratio could notbe measured.

As will be seen from Table 3, the difference between before and afterthe initialization about the reflectivity, CNR and erase ratio of eachof the samples of the examples 2-1 to 2-6 is small compared to thesamples of the comparative examples 2-1 and 2-2 which include nocrystallization assisting layer. It will be seen in each of the samplesof the examples 2-1 to 2-6 that its recording layer has beencrystallized by the related crystallization assisting layer and that itsrecording characteristics have been improved. It will be seen thatespecially in the examples 2-2 and 2-3 where the crystallizationassisting layer was crystallized before the recording layer was formed,there is little difference between before and after the initializationabout the reflectivity, CNR and erase ratio. Thus, it will be seen thatthe necessity for initialization process will be eliminated. In thesamples of the examples 2-1 to 2-3, the especially preferable resultsthat the CNR and the erase ratio were 53 and 25 dB or more,respectively, were obtained.

By comparison of the examples 2-3 to 2-5 and 2-7 to 2-10 which eachcomprise the Bi layer having the same thickness of 50 Å as thecrystallization assisting layer but which are different in thecomposition of the recording layer, it will be seen that since the ratiox:y:z indicative of the composition of the recording layer of each ofthe samples of the examples 2-7 to 2-10 does not satisfy the expressions(1)-(3) simultaneously, the recording layer is difficult to crystallizeand that the reflectivity before the initialization is very low comparedto the examples 2-3 to 2-5.

Results of X-Ray Diffraction

The orientations of crystals of the crystallization assisting layers andrecording layers of the X-ray diffraction samples A-C, produced asdescribed above, were examined in the X-ray diffraction apparatusmanufactured by K.K. Rigaku with a CuKα1 as the light source.

Since the crystal structure of the crystal phase of the phase changeoptical disk's recording layer (Ge—Sb—Te type) had a NaCl structure, thecrystal structure of the recording layer was given an index representingthe NaCl structure. Since Sb or Bi was used as the crystallizationassisting layer material, the crystal structure of crystallizationassisting layer was given an index representing a rhombohedralstructure. Giving an index representing a rhombohederal structure wasperformed by converting the rhombohederal structure to a hexagonallattice system, as is generally performed.

FIG. 3 is a chart of diffraction X-ray spectra where a vertical axisrepresents a diffraction intensity (I) and a horizontal axis a doubledvalue of a diffraction angle θ obtained from the respective samples A-C.

It was seen that when the sample C was directly subjected to X-raydiffraction, no diffraction peaks were observed and that its recordinglayer was amorphous. Thus, this sample was heated in an oven at atemperature of 275° C. for 10 minutes. Then, the resulting sample (newsample C) was again subjected to X-ray diffraction to obtain its spectrashown in FIG. 3.

It will be seen from this chart that the sample A (having itscrystallization assisting layer provided immediately under the recordinglayer) exhibits peaks in (111), (222) and (003) at angles of 2θ=25.7,52.9 and 23.7 degrees, respectively. It will be also seen that thesample B (including only the first dielectric layer and crystallizationassisting layer) exhibits peaks in (003) and (006) at angles of 2θ=23.7and 48.5 degrees, respectively. It will be further seen that the sampleC (having exactly the same structure as the sample A, excluding that nocrystallization assisting layer is provided) exhibits peaks in (200),(220) and (222) at angles of 2θ=29.7, 42.7 and 52.9 degrees,respectively.

Since the peaks in (111) (222) of the GeSbTe crystal appeared in theX-ray diffraction pattern of the sample A, it will be seen that therecording layer of the sample A was crystallized after its formation.

The orientation of Sb atoms which compose the crystallization assistinglayer can be known from the chart of the sample B. More particularly, itwill be known that powder Sb has high peak intensities in (012) and(104) whereas the Sb atoms of the Sb film formed on the first dielectriclayer are oriented in (003) and (006) planes.

By comparing the samples A and C, it will be known that the orientationsof their crystals differ depending on whether they include acrystallization assisting layer even when they have recording layers ofthe same composition and thickness. In this particular example, thepresence of the crystallization assisting layer causes peaks in the(200) and (220) to disappear.

Third Embodiment

EXAMPLE 3-1

A phase change optical disk having the same layer structure as the FIG.1 was produced as follows.

First, a first dielectric layer 2 having a thickness of about 100 nm wasformed on a polycarbonate substrate 1 having a diameter of 90 mm and athickness of 0.6 mm with a guide groove for a laser beam by RFsputtering with the aid of a target of ZnS—SiO₂. A Bi film having athickness of 3 nm was then formed as the crystallization assisting layer3 on the first dielectric layer 2 by sputtering with the aid of a targetof Bi.

A recording layer 4 of Ge_(20.3)Te_(56.7)Sb_(23.0) having a thickness of20 nm was formed on the crystallization assisting layer 3 by sputteringwith the aid of a target of a Ge—Te—Sb alloy. A second dielectric layer5 having a thickness of 12 nm was then formed on the recording layer 4by sputtering with the aid of a target of ZnS—SiO₂. A reflective layer 6having a thickness of 70 nm was then formed on the second dielectriclayer 5 by sputtering with the aid of a target of an Al alloy. A UVcurable resin was then spin-coated on the reflective layer 6 to be setto thereby form a UV cured resin layer 7.

The recording characteristics of the optical disk, thus obtained, weremeasured without initialization as follows: First, a laser beam having awavelength of 680 nm was modulated between a peak power of 12 mW and abias power of 5 mW. The sample of the optical disk was rotated at alinear velocity of 6 m/s while being irradiated with the modulated laserbeam to perform its initial recording and its CNR was measured. Therecording frequency was 5 MHz and the read power was 1 mW. The CNRs ofrecords on the optical disks were measured after the two, three and fouroverwriting or recording operations.

The CNRs measured after the one, two, three and four overwriting orrecording operations were 49.7, 48.8, 48.7 and 48.3 dB, respectively. Itwill be seen that the CNR decreased by 0.9 dB after the two recordingoperations.

The state of the sample present after the formation of itscrystallization assisting layer 3 and before the formation of itsrecording layer 4 was observed with the aid of a high resolution SEM(UHRSEM which was HITACHI S-5000 produced by Hitachi Seisakusho) at anacceleration voltage of 15 kV. A photograph of its reflective electronimage is shown in FIG. 4.

It will be seen from this photograph that the crystallization assistinglayer 3 is in the form of a discontinuous island-like film in which mostof the islands have a length of 8-40 nm with the longest one being 70nm. The interval between the islands was 20 nm at a maximum. Measurementby an atomic force microscope (AFM: Topo Metrix TMX-2000) in a contactmode clarified that an average thickness of the islands was 2.04 nm.

EXAMPLE 3-2

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 3-1 wasproduced, excluding that a Bi layer having a thickness of about 1.5 nmwas formed as the crystallization assisting layer 3.

The recording characteristics of the optical disk, thus obtained, weremeasured without initialization as in the example 3-1. The CNRs measuredafter the one, two, three and four overwriting or recording operationswere 51.7, 52.1, 51.8 and 52.0 dB, respectively. It will be seen that nodecrease occurred in the CNR after the two recording operations or more.

The state of the sample present after the formation of itscrystallization assisting layer 3 and before the formation of itsrecording layer 4 was observed with the aid of the high resolution SEMin a manner similar to that performed in the Example 3-1. A photographof its reflective electron image is shown in FIG. 5.

It will be seen from this photograph that the crystallization assistinglayer 3 is in the form of a discontinuous island-like film in which mostof the islands have a length of 4-20 nm with the longest one being 25 nmand that the interval between the islands was 13 nm at a maximum.Measurement similar to that of the Example 3-1 in a contact modeclarified that an average thickness of the islands was 1.73 nm.

EXAMPLE 3-3

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 3-1 wasproduced, excluding that a Bi layer having a thickness of about 0.65 nmwas formed as the crystallization assisting layer 3.

The measurement of the recording characteristics of the uninitializedoptical disk, thus obtained, clarified that its CNR was 50.8 dB afterrecorded first time, 51.1 dB after recorded twice, 51.3 dB afterrecorded three times, and 51.3 dB after recorded four times. That is, nodecrease in the CNR occurred after recorded twice or more.

EXAMPLE 3-4

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 3-1 wasproduced, excluding that a Bi layer having a thickness of about 0.3 nmwas formed as the crystallization assisting layer 3.

The measurement of the recording characteristics of the uninitializedoptical disk, thus obtained, clarified that its CNR was 54.0 dB afterrecorded first time, 54.1 dB after recorded twice, 53.9 dB afterrecorded three times, and 54.2 dB after recorded four times. That is, nodecrease in the CNR occurred after recorded twice or more.

Although the state of this sample present after the formation of itscrystallization assisting layer 3 and before the formation of therecording layer 4 was observed with the aid of the high-resolution SEMas in the example 3-1, no island-like objects could not be recognized.Since the resolution of this device was 0.7 nm, it was considered thatthe size of the island-like objects as the crystallization assistinglayer 3 was less than 0.7 nm. An average thickness of the island-likeobjects was measured as 1.01 nm in a manner similar to that used in theexample 3-1.

EXAMPLE 3-5

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 3-1 wasproduced, excluding that the first dielectric layer 2 had a thickness of115 nm, that the crystallization assisting layer 3 of Bi had a thicknessof 1.5 nm, that the recording layer 4 comprised aGe_(22.1)Te_(56.4)Sb_(21.5) layer having a thickness of 22 nm, and thatthe reflective layer 6 of an Al alloy had a thickness of 50 nm.

The measurement of the recording characteristics of the uninitializedoptical disk, thus obtained, performed in a manner similar to thatperformed for the example 3-1, clarified that its CNR was 53.2 dB afterrecorded first time, 53.3 dB after recorded twice, 53.2 dB afterrecorded three times, and 53.2 dB after recorded four times. That is, nodecrease in the CNR occurred after recorded twice or more.

EXAMPLE 3-6

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 3-1 wasproduced, excluding that a Bi layer having a thickness of about 5.0 nmwas formed as the crystallization assisting layer 3.

The measurement of the recording characteristics of the uninitializedoptical disk, thus obtained, clarified that its CNR was 50.7 dB afterrecorded first time, 47.5 dB after recorded twice, 47.9 dB afterrecorded three times, and 47.2 dB after recorded four times. That is, adecrease 3.2 dB in the CNR occurred after recorded twice.

When the state of this sample present after the formation of itscrystallization assisting layer 3 and before the formation of therecording layer 4 was observed with the aid of the high-resolution SEMas in the example 3-1, it was found that the crystallization assistinglayer 3 was formed in a continuous film.

Advantageous Effects Produced by the Crystallization Assisting Layer

It will be seen from the results of the examples 3-1 to 3-6 that whenthe crystallization assisting layer 3 of Bi was formed as adiscontinuous island-like film, the necessity for the initializationprocess for crystallizing the recording layer formed on thecrystallization assisting layer 3 was eliminated, and that the CNRsmeasured after the two or more recording operations by overwriting werenot lower than that measured in the initial recording operation.

It will also be known that the crystallization assisting layer 3 of Biwas formed so as to have a thickness of less than 3 nm to therebyprovide a discontinuous island-like film by general sputtering (when thesputtering conditions were: for example, the sputtering gas was Ar, thesputtering pressure was 0.5 Pa, and the power consumed was DC 10 W).

When the length of the island-like crystallization assisting layer isless than 100 nm, its discontinuous island-like film is easily obtainedby general sputtering which lacks patterning, for example. For theisland-like crystallization assisting layer 3, preferably, its length isless than 80 nm, and the maximum gap between two adjacent islands isless than 50 nm.

Crystal Structures of the Recording Layer and Crystallization AssistingLayer

An X-ray diffraction sample A was produced having the same layerstructure as the example 3-5, excluding that no UV cured resin layer 7was formed on its cover glass.

An X-ray diffraction sample B was produced having the same layerstructure as the sample A, excluding that no crystallization assistinglayer 3 was formed.

An X-ray diffraction sample C was produced having the same layerstructure as the sample A, excluding that the recording layer 4 had acomposition of Ge_(18.7)Sb_(26.9)Te_(54.4).

An X-ray diffraction sample D was produced having the same layerstructure as the sample A, excluding that the recording layer 4 had acomposition of Ge_(26.4)Sb_(17.6)Te_(56.0).

An X-ray diffraction sample E was produced having the same layerstructure as the sample A, excluding that the recording layer 4 had acomposition of Ge_(30.8)Sb_(12.9)Te_(56.3).

An X-ray diffraction sample F was produced having the same layerstructure as the sample A, excluding that the recording layer 4 had acomposition of Ge_(36.5)Sb_(9.3)Te_(54.2).

An X-ray diffraction sample G was produced having the same layerstructure as the sample A, excluding that the recording layer 4 had acomposition of Ge_(49.7)Te_(51.3).

Since the samples A, C-F each had a discontinuous island-like film of Bias the crystallization assisting layer 3, its recording layer 4 wascrystallized by the crystallization assisting layer 3. Since the sampleB lacked the crystallization assisting layer 3, its recording layer 4had not been crystallized. Thus, the sample B was heated at 275° C. for10 minutes in an oven to crystallize the recording layer. The sample B,thus obtained, was subjected to X-ray diffraction. The sample G had thecrystallization assisting layer 3, but its recording layer 4 was notcrystallized immediately after its formation.

Those X-ray diffraction samples A-G were each put on the X-raydiffraction apparatus to examine the orientation of the crystals of thecrystallization assisting layer and the recording layer. In this case,the X-ray diffraction apparatus produced by K.K. Rigaku, and CuKα1 asthe light source were used. Since the crystal structure of the phasechange optical disk recording layer (Ge—Sb—Te) in its crystalline phasewas a NaCl structure, the crystal structure of the recording layer wasregarded as having the NaCl structure and given a corresponding index.

FIG. 6 is a chart of the diffraction X-ray spectra of the respectivesamples A-G where the vertical axis represents a diffraction intensity(I) and the horizontal axis a doubled value of the diffraction angle θ.

It will be seen from this chart that the samples A-F exhibit peaks in(200) at 2θ=29.7 degrees whereas the sample G has no such peak. Theintensity of the peak tends to decrease as the Ge content of therecording layer increases and its Sb content decreases.

The sample B has peaks in (220) and (222) at 2θ=42.7 and 52.9 degrees,respectively, in addition to the peak in (200) at 2θ=29.7. This isconsidered to be due to the sample B lacking a crystallization assistinglayer 3. It will be seen that by forming a discontinuous island-likefilm of Bi as the crystallization assisting layer 3 and further forminga recording layer 4 of Ge—Sb—Te on the crystallization assisting layer3, the crystals of the recording layer 4 is easily oriented strongly ina surface (200) alone.

Fourth Embodiment

EXAMPLE 4-1

A phase change optical disk having the same layer structure as the FIG.1 was produced as follows.

First, a first dielectric layer 2 having a thickness of about 100 nm wasformed on a polycarbonate substrate 1 having a diameter of 90 mm and athickness of 0.6 mm with a guide groove for a laser beam andirregularities for address signals by RF sputtering with the aid of atarget of ZnS—SiO₂. A Bi layer having a thickness of 1.5 nm was thenformed as the crystallization assisting layer 3 on the first dielectriclayer 2.

A recording layer 4 of Ge₂₃Te₅₄Sb₂₃ having a thickness of 23 nm wasformed on the crystallization assisting layer 3. The temperature of thesubstrate 1 was 35° C. before the formation of the recording layer 4during which the temperature of the substrate 1 did not rise. A seconddielectric layer 5 of ZnS—SiO₂ having a thickness of 20 nm and areflective layer 6 of Al alloy having a thickness of 150 nm were thenformed sequentially on the recording layer 4 by sputtering. A UV curableresin was then spin-coated on the reflective layer 6 and set to therebyform a UV cured resin layer 7.

EXAMPLE 4-2

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 4-1 wasproduced, excluding that after the crystallization assisting layer 3 wasformed, the substrate 1 was irradiated with light from a halogen lamp toheat the substrate up to 75° C., and that the recording layer 4 was thenformed immediately. The temperature of the substrate was maintained in arange of from 45 through 75° C. inclusive during the formation of therecording layer 4.

EXAMPLE 4-3

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 4-1 wasproduced, excluding that after the crystallization assisting layer 3 wasformed, the substrate 1 was heated up to 55° C. with a heater. Thetemperature of the substrate was maintained in a range of from 45through 55° C. inclusive during the formation of the recording layer 4.

EXAMPLE 4-4

A phase change optical disk having the same layer structure as the FIG.1 was produced, excluding that immediately after the disk was formed upto the crystallization assisting layer 3 in a manner similar to that inwhich the example 4-1 was produced, a recording layer 4 of Ge₂₁Te₅₄Sb₂₅alloy having a thickness of 23 nm was formed, that the temperature ofthe substrate was 45° C. immediately after the formation of thecrystallization assisting layer 3, and that the temperature of thesubstrate was maintained 45° C. during the formation of the recordinglayer 4. The other conditions were all identical to those of the example4-1.

EXAMPLE 4-5

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 4-1 wasproduced, excluding that after the formation of the crystallizationassisting layer 3 the substrate 1 was irradiated with light from ahalogen lamp to heat the substrate up to 115° C., and that a recordinglayer 4 of Ge₂₃Te₅₃Sb₂₄ alloy having a thickness of 23 nm was thenformed immediately.

Effects Produced by the Heating of the Substrate when the RecordingLayer was Formed

The reflectivity and recording characteristics (CNR and erase ratio) ofeach of the optical disks of the examples 4-1 to 4-5, thus obtained,before the initialization were measured.

The reflectivity of each optical disk was measured, using a laser havinga wavelength of 680 nm immediately after the disk was obtained as suchand after its accelerating test was performed (in which the disk washeld within a tank of a humidity of 90% at a temperature of 80° C. for300 hours).

The recording characteristics of each disk were measured as follows.First, the laser beam having a wavelength of 680 nm was modulatedbetween an optimal peak power and an optimal bias power for each sample.By irradiating the optical disk sample which was rotating at 1800 rpmwith the modulated laser beam, it was subjected to initial recording(its recording frequency was 1.08 MHz), and then its CNR was measured.

Then, the initial recording signal recorded on the disk was overwrittenby a recording signal (having a frequency of 3.89 MHz) different fromthe initial recording signal (having a frequency of 1.08 MHz), and thenthe erase ratio of the initial recording signal was measured.

Those results are together shown in Table 4, attached hereto, on whichnumerical values appearing on the right of respective arrows in thereflectivity column are obtained immediately after the accelerationtest. The optical disk of the example 4-5 was measured by the measuringdevice, but address signals could not be read well in many places, andcannot be used.

As will be seen from Table 4, the reflectivitys of the optical disks ofthe examples 4-2 to 4-4 whose recording layers were formed at asubstrate temperature in a range of from 45 through 110° C. inclusivewere not substantially changed by the acceleration test. In comparativeexamples, the optical disks of the example 4-1 whose recording layer wasformed at a substrate temperature of 35° C. was insufficientlycrystallized. Thus, changes in its reflectivity due to the accelerationtest were found. The erase ratio of the optical disks of the examples4-2 to 4-4 were satisfactory compared to the optical disk of the example4-1.

As will be known from the above, in this embodiment, when the opticaldisks whose recording layer were formed at a substrate temperature in arange of from 45° C. through a substrate deforming temperature inclusivewere used without the initialization process, those optical disksexhibited stabilized characteristics compared to those disks whoserecording layers were formed at a substrate temperature of 35° C. Inmany cases, however, even when the recording layers were formed at asubstrate temperature of less than 45° C., the recording layers weresufficiently crystallized to thereby provide stabilized characteristics.

Fifth Embodiment

EXAMPLE 5-1

A phase change optical disk having the same layer structure as the FIG.1 was produced as follows.

First, a first dielectric layer 2 having a thickness of about 80 nm wasformed on a polycarbonate substrate 1 having a diameter of 120 mm and athickness of 0.6 mm with a guide groove for a laser beam by RFsputtering with the aid of a target of ZnS—SiO₂. A Bi layer having athickness of 1.5 nm was then formed as the crystallization assistinglayer 3 on the first dielectric layer 2 by sputtering with the aid of atarget of Bi.

A recording layer 4 of Ge₂₃Te₅₄Sb₂₃ having a thickness of 22 nm wasformed on the crystallization assisting layer 3 by sputtering with theaid of a target of a Ge—Te—Sb alloy. A second dielectric layer 5 havinga thickness of 12 nm was then formed on the recording layer 4 bysputtering with the aid of a target of ZnS—SiO₂. A reflective layer 6having a thickness of 100 nm was then formed on the second dielectriclayer 5 by sputtering with the aid of a target of an Al alloy. A UVcurable resin was then spin-coated on the reflective layer 6 and set tothereby form a UV cured resin layer 7.

During the formation of the recording layer 4, the temperature of thesubstrate was maintained at 85° C. The first dielectric layer 2,crystallization assisting layer 3, recording layer 4, second dielectriclayer 5 and reflective layer 6 were all formed within an atmosphere ofan argon gas.

EXAMPLE 5-2

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 5-1 wasproduced, excluding that the crystallization assisting layer 3 wasformed within an atmosphere of an argon gas plus a nitrogen gas of 4volume %.

EXAMPLE 5-3

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 5-1 wasproduced, excluding that the crystallization assisting layer 3 wasformed within an atmosphere of an argon gas plus a nitrogen gas of 8volume %,.

EXAMPLE 5-4

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 5-1 wasproduced, excluding that the crystallization assisting layer 3 wasformed within an atmosphere of an argon gas plus a nitrogen gas of 16volume %.

EXAMPLE 5-5

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 5-1 wasproduced, excluding that the crystallization assisting layer 3 wasformed within an atmosphere of an argon gas plus a nitrogen gas of 24volume %.

EXAMPLE 5-6

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 5-1 wasproduced, excluding that the crystallization assisting layer 3 wasformed within an atmosphere of an argon gas plus a nitrogen gas of 32volume %.

Effects Produced by Addition of Nitrogen to the CrystallizationAssisting Layer

The reflectivitys and recording characteristics of each of the opticaldisks of the examples 5-1 to 5-6 were measured before itsinitialization. First, the laser beam having a wavelength of 680 nm wasmodulated between a peak power of 11 mW and a bias power of 5 mWdepending on the recording signal. By irradiating the optical disksample rotating at a linear velocity of 6 m/s with the modulated laserbeam, the initial recording was performed. The resulting records on thedisk sample were read and its reflectivity and jitter were thenmeasured.

Next, the disk was overwritten by a recording signal, which producedrecords identical to the initial ones, 10, 1000, 20000, 30000, 50000,and 100000 times, the respective records were read, and then thecorresponding jitters were measured. The results of those measurementsare together shown on Table 5 attached hereto.

As will be seen from Table 5, the optical disks of the examples 5-2 to5-6 whose crystallization assisting layers were formed within theatmosphere containing a nitrogen gas were low in jitter value comparedto the optical disk of the example 5-1 example whose crystallizationassisting layer was formed within a nitrogen gas-free atmosphere, whenthe overwriting was performed repeatedly 20,000 times or more. Theoptical disks of the examples 5-2 to 5-6 are excellent in overwritecyclability compared to the optical disk of the example 5-1.

The crystallization assisting layer formed within the atmospherecontaining the nitrogen gas contains nitrogen, which is considered toimprove the overwrite cyclability.

The reason why the overwrite cyclability of the disks are improved dueto nitrogen being contained in the crystallization assisting layer canbe presumed as follows. The nitrogen contained in the crystallizationassisting layer gradually exudes out into the recording layer as therecording/erasing operations are repeated to change the recording layerpresent in the crystal state to a finer-crystal layer, to increase theviscosity of the recording layer present in the crystal state and toincrease the crystallizing temperature of the recording layer.Especially, it is considered that nitrides which will be formed on theinterface due to the changing of the recording layer to thefiner-crystal layer restrains the recording layer from partiallythinning due to movement of materials of the recording layer (flow ofthe materials of the recording layer in its direction of rotation duringmelting).

As a quantity of nitrogen contained in the crystallization assistinglayer increases, the overwrite cyclability improve. If the quantity ofnitrogen increases excessively, the crystallization becomes insufficientto thereby decrease the reflectivity. It is to be noted that since 15.5and 15.2% of the reflectivitys of the examples 5-5 and 5-6 are in an ausable range, there are no problems. The optical disks of the examples5-5 and 5-6 where the content of a nitrogen gas in the layer formingatmosphere was high have high jitter values compared to other opticaldisks after the recording was performed 1,000 times, but are less than15%, which is considered as not raising any questions practically.

Joint MORIS (Magneto-Optical Recording International Symposium/ISOM(International Symposium on Optical Memory) '97 Technical Digest p.292reported that addition of nitrogen to the recording layer greatlyimproved the overwrite cyclability.

The present invention is intended to eliminate the necessity for theinitialization process by providing a crystallization assisting layer onan optical information recording medium to crystallize the recordinglayer when the same is formed. When the recording layer of the opticalinformation recording medium having such crystallization assisting layercontains nitrogen, the recording layer cannot be crystallized whenformed.

Actually, a phase change optical disk having the same layer structure asthe FIG. 1 was produced in an manner similar to that in which theexample 5-1 was produced, excluding that the recording layer 4 wasformed within an atmosphere which contained an argon gas plus a nitrogengas of 4 volume %. The reflectivity of the disk measured before itsinitialization was 4.3%, and the recording layer was not becrystallized.

As described above, in an optical information recording medium having acrystallization assisting layer, the necessity for the initializationprocess is eliminated and the overwrite cyclability are improved byadding nitrogen to the crystallization assisting layer without addingnitrogen to the recording layer.

Sixth Embodiment

EXAMPLE 6-1

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 5-4 ofthe fifth embodiment was produced, excluding that the first dielectriclayer 2 was formed within an atmosphere which contained an argon gas andan oxygen gas of 0.1 volume %.

EXAMPLE 6-2

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 5-4 ofthe fifth embodiment was produced, excluding that the first dielectriclayer 2 was formed within an atmosphere which contained an argon gas anda nitrogen gas of 0.5 volume %.

EXAMPLE 6-3

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 5-4 ofthe fifth embodiment was produced, excluding that the second dielectriclayer 5 was formed within an atmosphere which contained an argon gas andan oxygen gas of 0.1 volume %.

EXAMPLE 6-4

A phase change optical disk having the same layer structure as the FIG.1 was produced in a manner similar to that in which the example 5-4 ofthe fifth embodiment was produced, excluding that the second dielectriclayer 5 was formed within an atmosphere which contained an argon gas anda nitrogen gas of 0.5 volume %.

Effects Produced by Addition of Oxygen or Nitrogen to the DielectricLayer

The reflectivitys and recording characteristics of the respectiveoptical disks of the examples 6-1 to 6-4 before their initializationwere measured in a manner similar to that used for the fifth embodiment.The results of those measurements are together shown in Table 6,attached hereto, which also contains the results of the measurement ofthe example 5-4 of the fifth embodiment for the sake of comparison.

As will be seen from Table 6, attached hereto, the jitter values of theoptical disks of the examples 6-1 to 6-4 which contain nitrogen in theircrystallization assisting layer 3 as well as oxygen or nitrogen in theirfirst or second dielectric layer are restricted to small ones comparedto the optical disk of the example 5-4 which contains only nitrogen inits crystallization assisting layer 3.

It will be seen that especially when the number of the overwriting islarge (for example, after recorded 100,000 times), the difference injitter between each of the disks of the examples 6-1 to 6-4 and arespective one of the optical disk of the example 5-4 is large, and thatthe overwrite cyclability of the disks are further improved byincorporating oxygen or nitrogen into the first or second dielectriclayer.

INDUSTRIAL APPLICABILITY

As described above, in an optical information recording medium obtainedby the inventive making method, the recording layer becomes crystallizedimmediately after its formation due to the existence of thecrystallization assisting layer of predetermined materials. Thus, thenecessity for the initialization process required in the past iseliminated. As a result, the process for making the optical informationrecording medium is simplified and the cost is reduced.

Especially, by forming the crystallization assisting layer in the formof a discontinuous island-like film of materials which comprise bismuth(Bi) and/or bismuth (Bi) compounds, an optical information recordingmedium having stabilized recording characteristics is obtained.

An optical information recording medium excellent in overwritecyclability is obtained which eliminates the necessity for theinitialization process by incorporating nitrogen into thecrystallization assisting layer.

TABLE 1 lattice material & thickness crystal structure B-valueunconformity Example PbTe face-centered cubic lattice system 4.5637 Å 5.519% 1-1 10 Å NaCl type <110> direction Example PbTe as above asabove as above 1-2 50 Å Example Bi₂Te₃ face-centered cubic lattice4.2931 Å −0.738% 1-3 10 Å system Bi₂Te₃ type <110> direction ExampleBi₂Te₃ as above as above as above 1-4 50 Å Comp. Ex. no crystallization— — — 1-1 assisted layer Comp. Ex. W body-centered cubic lattice 3.1653Å  −26.8% 1-2 10 Å system, body-centered cubic <100> direction latticeComp. Ex. Te hexagonal system selenium type 4.4579 Å  3.07% 1-3 30 Åa-axial direction Comp. Ex. Sb₂TeSe₂ rhombohederal lattice system  4.121Å −4.717% 1-4 30 Å Comp. Ex. Sb₂Te₃ rhombohederal lattice system 4.2463Å −1.809% 1-5 30 Å Comp. Ex. Ag₂Te monoclinic system  4.48 Å  3.58% 1-630 Å Comp. Ex. CrTe hexagonal system  3.98 Å  −9.13% 1-7 30 Å

TABLE 2 reflectivity CNR (dB) erase ratio (dB) initialization (%)once/100 times once/100 times Example before 18 50/51 22/24 1-1 after 2052/52 23/24 Example before 21 53/53 27/30 1-2 after 22 53/53 31/31Example before 18 49/50 28/30 1-3 after 21 52/52 30/30 Example before 2150/50 29/29 1-4 after 22 53/53 30/30 Comp. Ex. before 5 36/49  7/29 1-1after 20 53/53 30/30 Comp. Ex. before 6 33/49  7/28 1-2 after 22 50/5029/29 Comp. Ex. before 9 33/50  8/30 1-3 after 21 51/52 30/30 Comp. Ex.before 12 39/42  6/23 1-4 after 21 50/43 22/23 Comp. Ex. before 9 50/5120/20 1-5 after 18 50/51 21/20 Comp. Ex. before 8 38/46  6/22 1-6 after19 50/51 20/21 Comp. Ex. before 7 32/42  9/18 1-7 after 18 48/47 18/17

TABLE 3 initialization reflectivity CNR (dB) erase ratio (dB) Examplebefore 19 53 32 2-1 after 22 54 34 Comp. Ex. before 6 35  5 2-1 after 2153 35 Example before 20 53 27 2-2 after 21 54 27 Example before 20 54 322-3 after 22 54 33 Example before 22 49 25 2-4 after 23 53 27 Examplebefore 12 46 20 2-5 after 18 50 23 Example before 17 52 25 2-6 after 1856 27 Comp. Ex. before 5 45 17 2-2 after 17 53 25 Example before 6 40 122-7 after 17 47 17 Example before 4 unmeasurable unmeasurable 2-8 after15 unmeasured unmeasured Example before 4 unmeasurable unmeasurable 2-9after 13 unmeasured unmeasured Example before 4 unmeasurableunmeasurable 2-10 after 12 unmeasured unmeasured

TABLE 4 reflectivity (%) CNR (dB) erase ratio (dB) example 12 → 18 52 294-1 example 19 → 20 53 32 4-2 example 20 → 21 53 33 4-3 example 20 → 2154 32 4-4

TABLE 5 jitter (%) addition of N₂ after recorded in film reflectivity 101,000 20,000 30,000 50,000 100,000 formation (%) once times times timestimes times times example none 18.3 5.6 6.9 7.8 12.2 18.5 28.8 28.6 5-1example  4 vol % 18.7 5.7 7.2 7.8 11.1 14.5 23.2 28.1 5-2 example  8 vol% 19.2 5.8 7.4 7.8 9.2 12.1 18.7 22.7 5-3 example 16 vol % 18.6 6.3 7.58.3 8.8 11.1 11.9 16.9 5-4 example 24 vol % 15.5 7.5 7.5 10.0 8.5 9.110.1 14.5 5-5 example 32 vol % 15.2 7.5 7.7 10.6 8.9 9.3 10.2 13.2 5-6

TABLE 6 jitter (%) after recorded addition of N₂ or O₂ reflectivity 101,000 20,000 30,000 50,000 100,000 dielectric layer formation (%) oncetimes times times times times times example 1st dielectric layer 16.66.1 6.8 8.0 9.6 10.0 10.2 12.4 6-1 O₂ 0.1 vol % example 1st dielectriclayer 17.2 6.0 7.1 7.8 9.0 10.3 11.3 12.9 6-2 N₂ 0.5 vol % example 2nddielectric layer 17.6 6.2 7.6 7.9 8.8 11.2 11.3 13.2 6-3 O₂ 0.1 vol %example 1st dielectric layer 16.9 6.4 7.7 7.9 9.0 10.2 11.9 12.3 6-4 N₂0.5 vol % example no addition to dielectric 18.6 6.3 7.5 8.3 8.8 11.111.9 16.9 5-4 layer

What is claimed is:
 1. A method for producing an optical informationrecording medium having a recording layer of materials whose maincomponents comprise germanium (Ge), antimony (Sb) and tellurium (Te) onone side of a substrate comprising: forming on one side of the substratea crystallization assisting layer in the form of a discontinuousisland-like film of materials having a crystal structure of aface-centered cubic lattice system or of a tellurium (Te)-free materialhaving a crystal structure of a rhombohedral lattice system, an absolutevalue of lattice unconformity between the crystal structure of thecrystallization assisting layer and that of the recording layer beingnot more than 8%, the material of the crystallization assisting layercontaining Bi (bismuth) and/or a bismuth (Bi) compound, the thickness ofthe crystallization assisting layer being from 3 to less than 30 Å; andforming a recording layer by sputtering in a crystalline state withoutinitial crystallization directly over the crystallization assistinglayer.
 2. The method according to claim 1, wherein the recording layeris formed at a substrate temperature in a range of from 45° C. through atemperature inclusive above which temperature the substrate will bedeformed.
 3. The method according to claims 1, wherein the substrate isproduced of polycarbonate; and wherein the recording layer is formed ata substrate temperature in a range of from 45° C. through 110° C.inclusive.
 4. The method according to claims 1, wherein the formation ofthe crystallization assisting layer is performed within a film formingatmosphere to which a nitrogen gas is added.
 5. The method according toclaims 1, further comprising the steps of: forming a first dielectriclayer between the substrate and the crystallization assisting layer;and/or forming a second dielectric layer on an opposite side of therecording layer from the crystallization assisting layer, wherein theformation of the first and/or second dielectric layer is performedwithin a film forming atmosphere to which a nitrogen gas and/or anoxygen gas is added.
 6. An optical information recording mediumcomprising a recording layer formed on one side of a substrate bysputtering, the recording layer comprising materials whose maincomponents comprise germanium (Ge), antimony (Sb) and tellurium (Te),wherein the recording layer is formed in a crystalline state, suitablefor recording with laser beam, without initial crystallization; andcomprising: a crystallization assisting layer, for crystallizing therecording layer at the time of forming the recording layer, in the formof a discontinuous island-like film of materials having a crystalstructure of a face-centered cubic lattice system or of a tellurium(Te)-free material having a crystal structure of a rhombohedral latticesystem in contact with the substrate side surface of the recordinglayer, an absolute value of lattice unconformity between the crystalstructure of the crystallization assisting layer and that of therecording layer being not more than 8%, the material of thecrystallization assisting layer containing bismuth (Bi) and/or a bismuth(Bi) compound, the thickness of the crystallization assisting layerbeing from 3 to less than 30 Å.
 7. The optical information recordingmedium according to claim 6, wherein a ratio x, y, and z of the elements(Ge, Sb, Te) of the main components of the recording layer(Ge:Sb:Te=x:y:z where x+y+z=1) satisfies the following expressions(1)-(3) simultaneously: 0.1≦×≦0.4  (1) 0.08≦y  (2) 0.45≦z≦0.65  (3).